Minimizing Bandwidth Narrowing Penalities in a Wavelength Selective Switch Optical Network

ABSTRACT

This invention relates to provisioning wavelength-selective switches and reconfigurable optical add-drop multiplexers to minimize the bandwidth narrowing effect from the optical filters. Novel architectures and methods are disclosed that can significantly reduce bandwidth-narrowing on channels in a reconfigurable WDM network where a large number of optical filter elements are cascaded. Instead of blocking unused channels as in the prior art, unused channels are selectively provisioned depending on the state of their adjacent channels. Unused adjacent channels of an active channel are provisioned to follow the same path as the active channels. As each channels is deployed, the channel frequency is selected so as to minimize bandwidth narrowing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/019,255, filed, Feb. 1, 2011, entitled, “Minimizing BandwidthNarrowing Penalities in a Wavelength Selective Switch Optical Network”.

I. BACKGROUND

A. Field of Art

The present invention relates to the field of wavelength divisionmultiplexed (WDM) optical communication networks, and more particularlyto the architecture and operation of wavelength selective switches (WSS)and related devices to minimize bandwidth-narrowing penalties in suchnetworks.

B. Description of Related Art

Modern optical communication networks often employwavelength-multiplexed optical signals in a single transmission opticalfiber to increase the transmitted bandwidth. Such signals are typicallydeployed on a pre-defined frequency grid, such as the standard griddefined in ITU standard G.709. Each signal transmitted at one of thesestandard frequencies or wavelengths propagates throughout the network inits own distinct “channel” of that fiber. With such a grid, the centerfrequencies of adjacent channels are typically spaced at regularintervals, such as 50 GHz or 100 GHz. Alternatively, the centerfrequencies may be at arbitrary intervals, thereby forming an adjustablefrequency grid. In these systems, a wavelength-division multiplexer isused to combine a plurality of signals onto a single transmission fiber,with the frequency of each signal having a different nominal gridfrequency, and a wavelength-division demultiplexer used to separate thesignals at the end of the transmission fiber so that each signal isdirected to a distinct optical receiver. Each WDM signal is therebycapable of carrying separate and independent client traffic.

Optical networks may be configured in various topologies, such aspoint-to-point, ring, linear bus, or mesh. The topology employed in aparticular network is determined by the interconnections among the nodesand available fiber in that network. WDM networks may be deployed withfixed add/drop multiplexers, colorless add and drop couplers, and/orreconfigurable optical add/drop multiplexers (ROADMS). A ROADM at anetwork node may be constructed using one or more wavelength-selectiveswitches (WSS) configured to selectively add, drop, or block channelsbased on their grid frequency.

WSS technology is available today that supports more than 80 channelsthrough a single device typically having from 3 to 10 input/outputports. However, the optical technology can be extended to higher channelcounts and port counts. Several types of WSS optical modules have beenproposed (see, e.g. U.S. Pat. Nos. 7,492,986 and 6,487,334).

Regardless of the particular technology employed, a WSS typically hasthe ability to selectively direct a signal from an input port to anyoutput port based on the frequency (or wavelength) of the signal. Theroute or path of a signal originating at a source node of the network,and passing through one or more intermediate nodes before reaching adestination node, may be deemed to include its path within a node aswell (i.e., between one or more WSS input ports and one or more WSSoutput ports).

A ROADM node may also have: (1) a channel monitor that monitors thepower at each frequency grid point; and, (2) a means of attenuating thepower of each channel transmitted in a fiber. The channel monitor andpower adjustment may be integrated into the WSS module or implemented asseparate modules. Regardless of the particular implementation, thecombination of a channel monitor with power control enables thefunctions of (1) balancing the channels at one or more points in theROADM node and (2) selectively blocking channels by maximizing theirattenuation.

WSS technology, coupled with a management overhead channel, enablesremote network reconfiguration from a central network operations center(NOC). The management channel can be transmitted over an external IPnetwork, a dedicated optical service channel, or within the embeddedoverhead of an optical signal.

In a typical deployment, before any channels have been added to thenetwork, all channels of the WSS are set at full attenuation, which canbe referred to as the blocking state or “B” state. This preventsamplified spontaneous emission (ASE) from optical amplifiers frompropagating and being amplified through the network when a particularchannel is not present in the network. Circulating ASE is of particularconcern in networks with a closed optical path, such as ring topologies,because of the optical power instability it can cause. In an amplifiednetwork with one or more closed paths (such as in a ring network), eachgrid channel is typically blocked or dropped at least once to preventASE instability.

If a channel is being reused, i.e. the same channel frequency is beingreused by two or more non-overlapping separate signals, then the lightfrom the first signal must be effectively blocked before the secondsignal is added so as to prevent cross-talk penalties. Dropped signalsare not blocked in broadcast applications, however, because that samesignal must propagate to the other nodes receiving the broadcast signal.

A WDM network is typically deployed with a “guard-band” between thenominal frequencies. A guard band is required because: (1) practicaloptical filters used in WSS modules have a finite slope between theirpass bands and stop bands; (2) optical signals have a modulationbandwidth on the order of their bit rate; and, (3) errors occur in laserfrequencies and center frequencies in optical filters due tomanufacturing tolerances, calibration errors, temperature drifts, andcomponent aging. For example, 100 GHz channel spacing may be used forchannels at 10 Gb/s or 40 Gb/s, which have full-width at half-maximumbandwidth less than 50 GHz. As optical networks have evolved, themaximum bit rate has increased, with 100 Gb/s networks currently beingdeployed, with a reduced grid frequency spacing of 50 GHz. Thus, therelative guard band is decreasing over time while requirements onfrequency accuracy are increasing.

A significant design issue for WSS filters is the problem of bandwidthnarrowing. As client signals traverse WSS modules in a network whereeach WSS is set to attenuate unused adjacent channels (e.g., unusedchannels at 193.9 THz and 194.1 THz adjacent to signal channel 194.0THz), the effective passband of the WSS cascade is reduced, which canlead to bit errors. For an optical signal in a particular channelpropagating through a WSS network, a bandwidth narrowing event occurs ateach WSS where one or both of the channels adjacent to the signal areset to a different physical state (e.g., “pass through” as compared to“blocking” or “add”) than the state of the channel of the given signal.

All optical filters have a useable passband which is less than that ofan ideal filter due to the finite slope of a manufacturable filterpassband. Moreover, the useable bandwidth of cascaded filters decreasesas more filters are inserted in the signal path. This bandwidthnarrowing effect has led WSS designers and manufacturers to increase theeffective Gaussian order of the WSS pass band spectral shape [See forexample “Wavelength-Selective Switches for ROADM Applications” in IEEEJournal of Selected Topics in Quantum Electronics, vol 16, pp.1150-1157, 2010]. Such techniques have improved, but not eliminated, theproblem of bandwidth narrowing. Therefore, as the bit rate (and hencebandwidth) of optical signals increases, and the size of ROADM networksincrease, there remains a need for more effective techniques ofminimizing WSS bandwidth narrowing.

Accordingly, a solution is desired that provisions channels carryingclient signals in WSS modules so as to minimize bandwidth narrowingwhile still preventing significant ASE circulation and coherentcross-talk among different transmitters operating at the samefrequencies.

II. SUMMARY

In accordance with the present invention, various embodiments of novelmethods and architectures are disclosed for operating wavelengthselective switch devices and/or other bandwidth narrowing devices in awavelength division multiplexed optical network. In one embodiment, eachWSS device in the network maintains a provisioned state for each WDMoptical channel. As is the case with existing networks, concernsrelating to ASE circulation and cross-talk warrant assigning an initialdefault blocking state (“B”) to all channels on all ports.

At any given node, multiple client signals can be added to the networkas well as dropped from the network. As noted above, one or more WSSmodules at each network node can include multiple input and outputports, and permit a client signal on any input port to be routed to anyone or more output ports based on its channel wavelength or frequency(where signals on each output port can propagate along one or moreoptical fibers following distinct routes or paths among differentnetwork nodes). Moreover, a WSS module can be configured in a filteredadd configuration to selectively add client signals to the network, orin a filtered drop configuration to selectively drop client signals fromthe network. In each of these configurations, the WSS can alsoselectively attenuate each signal by a programmable amount. Theinventive concepts discussed herein apply equally to all such scenarios.

For the sake of simplicity, however, we will focus herein on “degree 2”nodes in which a WSS selects among 2 input ports per channel—input port“1” representing a client signal transmitted to that node from anothernode, and input port “2” representing a client signal originating (i.e.,being added) at that node. The WSS routes a channel to its output port(a single output port in a degree 2 node) from input port 1 or inputport 2 (while blocking the same signal from the other input port), or itblocks the channels on both of its input ports (while optionally alsodropping the channel from input port 1 to a receiver at the node).Again, in each of these configurations, each WSS can provide distinctattenuation to each signal.

The WSS at a given node thus maintains one of three physical states foreach channel: (1) a physical “pass through” state, logically representedherein as “PT-1” in which the channel on input port 1 passes through theWSS (and the node) to another node in the network, possibly with aprogrammable attenuation; (2) a physical “add” state, logicallyrepresented herein as “PT-2” in which the channel on input port 2 isadded to the network from this “source” node, possibly with aprogrammable attenuation, and propagated to another node in the network;or (3) a physical “blocking” state, logically represented herein as “B”(or “BD” for the case in which the channel is dropped at this“destination” node), which corresponds to a substantial attenuation ofthe signal toward the passthrough port.

In other embodiments, multiple states could be employed at a node on agiven channel—e.g., utilizing multiple output ports to “pass through” achannel to one output port connected to another node via one opticalfiber while “blocking” that channel on a second output port connected toa different node via a second optical fiber. In such embodiments, forexample, each output port might have its own state per channel.

Until a client signal is provisioned on a given channel, the state ofthat channel at all nodes remains the default blocking state (logicalstate “B”). When a client signal is added at a node and provisioned on aparticular channel, the state of that channel at that “source” nodetransitions to the physical “add” state, represented herein as thelogical “PT-2” state, while the state of that channel at each“intermediate” node along its route transitions to the physical “passthrough” state, represented herein as the logical “PT-1” state, and thestate of that channel being dropped at its “destination” nodetransitions to the physical “blocking” state, represented herein as thelogical “BD” state. Note that the logical state table representing thephysical configuration of each WSS will have a separate entry for eachchannel at each WSS module.

It should be noted that, in one embodiment, the logical “PT-2” statetakes priority over the “BD” state for this scenario in which a channelis reused. Note that, with programmable add WSS modules, the PT-2 statefor a particular channel implies that the same channel is blocked on theWSS input port 1. In other words, the node is both a “source” node for anew client signal added on a particular channel, and a “destination”node for another client signal (from another node) dropped (received) onthat same channel. The fact that the signal is also dropped at the nodecan be inferred from the “PT-2” state, as well as known via other meansof communication among the nodes.

Bandwidth narrowing of a particular signal traversing a WSS occurswhenever the WSS is set to block the adjacent signal frequencies alongthe same path. However, if the WSS is set to direct the adjacent signalfrequencies along the same path as the given signal, then bandwidthnarrowing does not occur. Therefore, to address the bandwidth narrowingissue that results when client signals provisioned on any particularchannel propagate through multiple WSS modules at multiple networknodes, the present invention considers the state of adjacent channelswhenever a new client signal is added at a node (including the state ofadjacent channels at intermediate nodes before the signal is dropped atits destination node). If either or both of those adjacent channels isunused (i.e., in a “B” state, as in the prior art), then each suchunused adjacent channel is “unblocked” and transitions to a new logicalstate (“PTA-1” for a channel adjacent to a “PT-1” channel, and “PTA-2”for a channel adjacent to a “PT-2” channel) having the same physicalstate as the client signal's channel (or “signal channel”) to which itis adjacent.

For example, if a new client signal is added at a node, the “B” state ofany unused adjacent channel is changed to the “PTA-2” state (and to the“PTA-1” state at each intermediate node before the signal is dropped atits destination node). As a result, the unused adjacent channels havethe same physical state (albeit not the same logical state) as does thesignal channel. This is true at the channel's source node (“add” state),at each intermediate node (“pass through” state) and at the channel'sdestination node (“blocking” state).

Although no client signal is present on these unused adjacent channels,these channels are no longer “blocked.” As a result, the effectivepassband of each such client signal is widened throughout the cascade ofWSS filters to encompass the unused adjacent channels in addition to thesignal channel. This in turn reduces the number of bandwidth narrowingevents along the route of the client signal, and thus reduces theprobability of bit errors. Even though the unused channels are notblocked at each WSS, they are still blocked at the WSS that drops theparticular signal. So, the WSS at the drop site still preventssignificant ASE circulation and coherent cross-talk among differenttransmitters operating at the same frequencies.

In one embodiment of the present invention, the provisioning commandsfor a given channel are sent to each WSS over a network managementchannel, and the WSS sets the state of the adjacent channelsaccordingly. In another embodiment, the provisioning commands for theprovisioned channel and adjacent channels are sent to each WSS over anetwork management channel. In yet another embodiment, a channel monitoris employed at each WSS to monitor the signal channels and provision theWSS to add channels when the monitored channel power reaches a minimumthreshold, at which point the unused adjacent channels are alsoprovisioned to follow the same path as the detected signal channel.

The embodiments disclosed herein apply to provisioning each WSS inmulti-degree nodes architected by cascading WSS modules, and areapplicable to virtually any WDM channel plan and virtually any networkarchitecture. These techniques also permit reuse of channels fornonoverlapping signal paths (in one embodiment, via two transmittersoperating at the same frequency). Moreover, an algorithm is alsodisclosed for selecting new channel frequencies so as to minimizebandwidth narrowing penalties.

III. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a degree two ROADM node with a filtered addconfiguration where a 2×1 WSS is configured to selectively add signalsto the optical network;

FIG. 2 is a block diagram of a degree two ROADM node with a filtereddrop configuration where a 2×1 WSS is configured to selectively dropsignals from the optical network;

FIG. 3 illustrates an optical ring network with seven ROADM nodesselected to transmit Channel 194.0 (at frequency 194.0 THz) from Node 1to Node 5 in the counterclockwise direction. Each ROADM node has the WSSconfiguration shown in FIG. 1.

FIG. 4 is a 5-channel subsection of a table that lists the conventionalconfiguration of each channel of every counterclockwise WSS for thenetwork of FIG. 3.

FIG. 5 is a graph illustrating the bandwidth narrowing effects for theprovisioning example in FIG. 4.

FIG. 6 is a 5-channel subsection of a table that lists the novelconfiguration of each channel of every counterclockwise WSS for thenetwork of FIG. 3.

FIG. 7 is a graph illustrating the reduced bandwidth narrowing effectsfor the provisioning example in FIG. 6.

FIG. 8 illustrates the optical ring network of FIG. 3 for the case wherechannel 193.9 (193.9 THz) is transmitted from Node 4 to Node 2 in thecounterclockwise direction.

FIG. 9 is a 5-channel subsection of a table that lists the conventionalprovisioning of each channel of every counterclockwise WSS for thenetwork of FIG. 8.

FIG. 10 is a 5-channel subsection of a table that lists the novelprovisioning of each channel of every counterclockwise WSS for thenetwork of FIG. 8.

FIG. 11 is a state table that lists the novel provisioning states foreach WSS in an optical network, reflecting the state changes of achannel that is added to the WSS network as well as those of thechannels adjacent to the added channel.

FIG. 12 is a flowchart illustrating one embodiment of an algorithm forselecting new channel frequencies so as to minimize bandwidth narrowingpenalties.

IV. DETAILED DESCRIPTION

Embodiments of the present invention described below includearchitectures and methods of provisioning WSS modules in an opticalnetwork so as to minimize bandwidth narrowing effects while preventinginstabilities from circulating ASE and enabling channel reuse. In theseembodiments, the operation of the WSS devices is described in thecontext of a ring WDM optical network which supports a plurality ofchannels with central frequencies on a pre-defined grid of 100 GHz, suchas 193.0 THz, 193.1 THz, etc. However, it will be appreciated that theparticular channel spacing used in these embodiments can vary, and thateven uniform channel spacing is not essential. Moreover, while theseembodiments employ a “degree 2” WSS network in a ring topology, thepresent invention is equally applicable to more generalized topologiesand higher-degree WSS nodes.

FIG. 1 illustrates one embodiment of a reconfigurable optical node 100that uses a WSS module 110 in a filtered add configuration where the WSSis positioned to add signals from an add input port 123 to the opticalnetwork, and pass or block the incoming signals from the passthroughinput port 112. The WSS 110 has a control port 116 that is connected toa CPU 113 running software that sends provisioning information to theWSS in the form of a table that specifies the desired state for eachoptical channel. Node 100 also may include an integrated or separatechannel monitor 114 that reports the power in each channel to the CPU113. For a WSS with per-channel power control, the CPU 113 may use thereported channel powers from channel monitor 114 to adjust the targetchannel powers of the WSS. This per-channel power control may be used tobalance the channels at the line output fiber 118. The CPU 113 caninclude a suitably programmed microprocessor or the like, and mayreceive provisioning information from a network manager 117 via itsexternal management port 115.

Signals from transmitters 115 may be added to the network at node 100 bymultiplexing them with a standard optical multiplexer 120 (e.g.,wavelength specific and/or colorless) and then directing the multiplexedsignals to one of the WSS ports, add input port 123 in this example. Inother embodiments, WSS 110 may have more than two input ports, e.g. N+1input ports, so that N transmitters may be directly connected to the WSSinput ports. Signals entering the node from the line input fiber 125 maybe dropped with a 1×2 optical coupler 140 that sends a portion of theincoming power (via output port 142) to an optical demultiplexer 130,which separates the signals based on their channel frequency. At theoutput ports 135 of demultiplexer 130, the received signals areconnected to a plurality of optical receivers 150. the demultiplexer 130can be wavelength specific, colorless, WSS, etc.

The other output port of optical coupler 145 is connected to WSSpassthrough input port 112. Signals entering node 100 from line inputfiber 125 are thereby directed to WSS 110 which selectively controls thesignals based on their frequency to selectively (1) pass the signal fromline input fiber 125 to line output fiber 118 (via passthrough inputport 112 and WSS output port 111) and block the signal entering from addinput port 123; (2) block the signal from line input fiber 125 and passthe signal, preferably with controlled attenuation, from add input port123 to line output fiber 118 (via WSS output port 111); or (3) block thesignals entering from both input ports 112 and 123. If per-channelattenuation functionality is available in the WSS devices, then theattenuation of each channel can be set such that all signal powers areset to be equal at the line output 118.

Note that the embodiment of FIG. 1 shows only one direction (Left toRight) of signal propagation with a single input line fiber and a singleoutput line fiber. While optical networks are typically implemented witha fiber pair between nodes, we show only one direction in our examplesfor simplicity of illustration. Also note that an optical node may haveother elements such as amplifiers, tap couplers and monitors, opticalservice channel taps and transceivers, dispersion compensation modules,etc. Again, for the purpose of simplification, we do not show suchelements in our examples, but it is understood that they may be deployedas desired. Further note that the embodiment of FIG. 1 only shows a nodeof degree two, while those skilled in the art will recognize that thesesame concepts would be applicable to higher-degree nodes.

FIG. 2 illustrates another embodiment of a ROADM node 200, in a filtereddrop configuration where WSS 210 is positioned to selectively dropoptical signals to one or more drop ports 220 and selectively passthrough optical signals, preferably with controlled attenuation, to lineoutput fiber 230. The concepts described herein could apply equally toROADM node configuration 100 or 200, as well as to higher-degree ROADMnodes. For purposes of illustration, the following embodiments willrefer to the embodiment shown in FIG. 1. Note also that the embodimentsdescribed herein refer to a channel plan with 100 GHz spacing, such thatthe 193.9 THz channel, for example, is adjacent to the 194.0 THzchannel. Those skilled in the art will recognize that the conceptsdescribed herein are equally applicable to other channel spacings andother node architectures employing WSS and other opticalbandwidth-narrowing devices.

Note that the embodiment of FIG. 2 shows only one direction (Right toLeft) of signal propagation with a single input line fiber and a singleoutput line fiber. While optical networks are typically implemented witha fiber pair between nodes, we show only one direction in our examplesfor simplicity of illustration. Also note that an optical node may haveother elements such as amplifiers, tap couplers and monitors, opticalservice channel taps and transceivers, dispersion compensation modules,etc. Again, for the purpose of simplification, we do not show suchelements in our examples, but it is understood that they may be deployedas desired. Further note that the embodiment of FIG. 2 only shows a nodeof degree two, while those skilled in the art will recognize that thesesame concepts would be applicable to higher-degree nodes.

FIG. 3 depicts a seven-node optical network 300 in a ring configurationwith the node configuration shown in FIG. 1. Network 300 shows a singleline fiber 305 transmitting signals among nodes 310, 320, 330, 340, 350,360, and 370 in a counterclockwise direction. When no channels have beenprovisioned, e.g. when the network is first installed, the controlsoftware sets the WSS in each node to block all the other channels toprevent circulating noise from optical amplifiers (not shown). In thisexample, channel 194.0 (194.0 THz) is provisioned to be added at Node 1310 and propagate to Node 5 350 in the counterclockwise direction.Network manager software 380 is used to monitor and provision the WSS ateach node, preferably using the overhead channel mentioned above.

The WSS at Node 1 310 is configured to add channel 194.0 at its inputport 2 (state “PT-2”); the WSS at the intermediate nodes 2-4 320, 330,and 340 are set to pass channel 194.0 from their first input port to theoutput (state “PT-1”); the WSS at Node 5 350 that is dropping channel194.0 is set to block that channel (state “BD”); and the WSS at Nodes 6and 7 360 and 370 block channel 194.0 at all ports (“B”). Note that wedifferentiate the state where a WSS is blocking a signal that is notpresent at its node (state “B”) from the state where a node is blockinga signal that is dropped at its node (state “BD”).

The Table 400 in FIG. 4 shows the conventional provisioning (prior art)of the WSS modules for the example shown in FIG. 3. Note that, for thepurpose of simplicity, this table shows only a subsection of thechannels (Chs. 193.8 to 194.2) in the network. As described above,column 410 of Table 400 lists the state of each WSS in the network forChannel 194.0. All other channels are blocked, denoted by the “B” statein Table 400; in particular the channels adjacent to Channel 194.0 areset to the “B” state along the path of the Channel 194.0, denoted by theshaded entries 420 in Table 400. The setting of the channels adjacent tochannel 194.0 to a blocked state will cause spectral narrowing onchannel 194.0 as it propagates from Node 1 to Node 5.

As shown by the shaded entries 420, Channel 194.0 will experience thisspectral narrowing at 4 edges on its lower frequency edge from theblocked Channel 193.9 at Nodes 1-4, and at 4 edges on its higherfrequency edge from the blocked Channel 194.1 at Nodes 1-4. These 8bandwidth narrowing events are represented by the differences inphysical states between the signal channel (Channel 194.0) and itsunused adjacent channels (Channels 193.9 and 194.1). For example,Channels 193.9 and 194.1 are in the “blocking” state at Nodes 1-4,whereas Channel 194.0 is in the “add” state at Node 1 and in the “passthrough” state at Nodes 2-4. Note that, despite the difference inlogical states at “destination” Node 5, Channel 194.0 (“BD”) is in thesame physical “blocking” state (“B”) as are Channels 193.9 and 194.1,indicating the lack of a bandwidth narrowing event. Thus, this prior artconfiguration, with a cascade of 5 ROADMs, results in significantbandwidth narrowing.

Graph 500 in FIG. 5 illustrates the spectral narrowing experienced bythe channel 194.0 passband as it propagates through each WSS with theprovisioning illustrated in FIG. 4. Since the adjacent channels at 193.9THz and 194.1 THz are blocked, the passband of a single WSS has the losscurve 510. Loss curves for two WSS modules 520, three WSS modules 530,and four WSS modules 540 show that the passband of the 194.0 THz channelis further narrowed as it passes through each WSS module whilepropagating from Node 1 to Node 4 with the conventional provisioningshown in table 400 of FIG. 4. This spectral narrowing through a cascadeof WSS modules with conventional provisioning is well known to thoseskilled in the art.

Table 600 in FIG. 6 illustrates a novel method of provisioning the WSSmodules for the example shown in FIG. 3. Column 610 of Table 600 liststhe state of each WSS in the network for Channel 194.0. Note that theseare the same states as listed in column 410 of Table 400. Channels thatare not adjacent to Channel 194.0, such as 193.8 and 194.2, remainblocked as shown in columns 620. However, channels that are adjacent tothe provisioned Channel 194.0 are provisioned differently from theconventional method shown in Table 400.

In the example illustrated in FIG. 6, each WSS sets the adjacentchannels to the same physical state as the provisioned channel (“add” inNode 1, and “pass through” in Nodes 2-4), as shown in columns 630. Thismay include setting the adjacent channels' attenuation to the sameattenuation as that for the provisioned channel. Even though no clientsignals are present on adjacent channels 193.9 and 194.1 (only onChannel 194.0), the WSS at Node 1 is configured to add the adjacentchannels at its input port 2 (state “PTA-2”); the WSS at intermediatenodes 2-4 are set to “pass through” the adjacent channels from theirfirst input port to the output (state “PTA-1”); and the WSS at Node 5that is dropping the 194.0 client signal is set to block the adjacentchannels (state “B”); and the WSS at Nodes 6 and 7 continue to block theadjacent channels at all ports (state “B”).

Note, as will be explained in greater detail below, that wedifferentiate the state where a WSS is passing a provisioned signal fromport N (state “PT-N”) from the state where a WSS is set to pass achannel from port N that is adjacent to a signal channel (state“PTA-N”). Also note that the node passthrough path of the adjacentchannels is still blocked at at least one node (Nodes 5, 6, 7 and 1 inthis case), so that recirculating ASE will not occur at the adjacentchannels.

Since the unused adjacent channels of the signal in channel 194.0 areset to propagate along the same path as the signal, the signal does notexperience any spectral narrowing as it propagates through the WSSmodules. This is reflected in the fact that, despite the difference inlogical states, the physical states of adjacent Channels 193.9 and 194.1are the same as those of signal Channel 194.0 (“add” in Node 1, and“pass through” in Nodes 2-4).

Spectral filtering at the edge of the signal channel only occurs at theadd multiplexer (element 120 in FIG. 1) and the drop demultiplexer(element 130 in FIG. 1). In this example, the number of WSS bandwidthnarrowing occurrences has been reduced from eight (2 edges at each offour locations—Nodes 1-4) to zero. In larger networks with more nodes,this reduction of spectral narrowing can be even more significant.

Graph 700 in FIG. 7 illustrates the passband of the WSS modules for thenovel provisioning shown in FIG. 6. Since the unused adjacent channelsare provisioned to the same physical state as is signal channel 194.0(preferably including per-channel attenuation), the bandwidth narrowingoccurs only on the edges of the adjacent channels, and the 194.0 signalchannel does not experience spectral narrowing on its spectral edges asit propagates through the WSS modules.

This reduction of bandwidth narrowing depends on the usage of theadjacent channels. WSS bandwidth narrowing on a particular signal occursonly when an adjacent channel is present and blocked (and/or set at asignificantly different value of attenuation), or configured along adifferent path through the WSS. Whenever an adjacent channel is blockedbecause it is not present, the bandwidth-narrowing penalty resultingfrom conventional methods can be eliminated by the present invention.

For WSS modules with a power control function where the unused adjacentchannels are provisioned as disclosed herein, it is preferable that theunused adjacent channels be attenuated to the same levels as are theprovisioned channels. Low-power alarms for the unused adjacent channelsshould also preferably be disabled.

In one embodiment, provisioning of channels using the newly introduced“PTA-N” state employs a hierarchy to appropriately provision the WSSwhen adjacent channels are activated. This hierarchy is illustratedusing network 800 of FIG. 8, which shows the network of FIG. 3 when theadditional Channel 193.9 is provisioned to be transmitted from Node 4 toNode 2 in the counterclockwise direction.

Table 900 in FIG. 9 illustrates the conventional provisioning (priorart) of the WSS modules for the example shown in FIG. 7. Theprovisioning of Channel 194.0 in column 910 is the same as in Table 400in FIG. 4. Column 920 in FIG. 9 shows the state of each WSS module forthe added Channel 193.9. The WSS at the Channel 193.9 transmitter node,Node 4, is set to pass the Channel 193.9 signal from the second inputport (state “PT-2”), and the WSS at the intermediate nodes, Node 5, 6,7, and 1 are set to pass the Channel 193.9 signal from the line inputport to the line output port (state “PT-1”). The node that is droppingthe signal, Node 2, is set to block Channel 193.9 (state “BD”); and Node3, which is not in the added signal's path, is left in the blockingstate (“B”).

In this example of conventional provisioning shown in Table 900, theoriginal channel, 194.0, still passes through 4 WSS modules that effectbandwidth narrowing on its spectral edges, as is apparent from acomparison of column 910 (for Channel 194.0) to both column 920 (foradjacent Channel 193.9) and column 930 (for adjacent Channel 194.1).With respect to the path of the signal on Channel 194.0, from Node 1 toNode 4, the adjacent channels have a different physical state thanChannel 194.0 at 8 edges (2 edges in each of 4 WSS locations). Inparticular, at Node 1, signal Channel 194.0 has a different physicalstate (“add”) from that of adjacent Channels 193.9 (“pass through”) and194.1 (“blocking”). At Nodes 2 and 3, signal Channel 194.0 has adifferent physical state (“pass through”) from that of adjacent Channels193.9 (“blocking,” despite a different logical “BD” state) and 194.1(“blocking”). Finally, at Node 4, signal Channel 194.0 has a differentphysical state (“pass through”) from that of adjacent Channels 193.9(“add”) and 194.1 (“blocking”).

Also, in this example of conventional provisioning shown in Table 900,the second channel, 193.9, passes through 5 WSS modules (in Nodes 4, 5,6, 7 and 1) that effect bandwidth narrowing on its specral edges, as isapparent from a comparison of column 920 (for Channel 193.9) to bothcolumn 940 (for Channel 193.8) and column 910 (for Channel 194.0). Thus,in the conventional provisioning example of FIG. 9, the addition ofChannel 193.9 does not change the bandwidth narrowing on Channel 194.0.Similarly, the addition of Channel 194.0 does not change the bandwidthnarrowing on Channel 193.9.

Table 1000 in FIG. 10 illustrates a novel method of provisioning the WSSmodules for the example shown in FIG. 8. The provisioning of Channel194.0 in column 1010 is the same as in the previous example shown inFIG. 6. The provisioning of the added channel 193.9 has been modified inthe same manner as described above at the nodes that add, drop, andpassthrough the added channel 193.9, namely Nodes 4, 5, 6, 7, 1 and 2.Node 3, which is not in the added signal's path, is left in the previous“PTA-1” state.

Note that the new WSS states of “PT-1”, “PT-2”, and “BD” for Channel193.9 that create the new signal path (shown in column 1030) overwritethe previous states (shown in FIG. 6) that created a path in thischannel adjacent to Channel 194.0. Also note that creating the new pathfor channel 193.9 results in changes to the WSS states of the unusedchannel 193.8 (now shown in column 1040), which is provisioned as anunused channel adjacent to the new signal in Channel 193.9. Provisioningthe new path for Channel 193.9 also affects the provisioning of adjacentChannel 194.0 at Nodes 6 and 7, which have their state changed from “B”to “PTA-1” so as to reduce the bandwidth narrowing on the Channel 193.9signal. Note, however, that the state of Channel 194.0 at the othernodes is not modified by the addition of the new signal at channel193.9.

Thus, as a result of provisioning a new signal on Channel 193.9, theoriginal signal on Channel 194.0 now experiences some bandwidthnarrowing events (as illustrated by the 3 shaded entries in column1030), but far fewer than the 8 bandwidth narrowing events shown in theconventional provisioning example of FIG. 9.

For example, with respect to Node 1, signal Channel 194.0 has adifferent physical state (“add”) from that of adjacent Channel 193.9(“pass through”), but the same physical state as adjacent Channel 194.1.Similarly, with respect to Nodes 2 and 4, signal Channel 194.0 has adifferent physical state (“pass through”) from that of adjacent Channel193.9 (“blocking” and “add,” respectively), but the same physical stateas adjacent Channel 194.1. And, with respect to Node 3, Channel 194.0has the same physical state (“pass through”) as both adjacent Channels193.9 and 194.1. At the same time, in accordance with the presentinvention, the states of Channel 194.1 have changed in Nodes 1, 2, 3 and4, the state of Channel 193.8 has changed in Node 3, and the states ofChannel 193.8 have changed in Nodes 1, 4, 5, 6 and 7 (as compared toconventional provisioning shown in FIG. 9).

FIG. 11 illustrates one embodiment of a set of rules for changing thestate of the WSS at each node when a signal is added. Note that theserules assume that the channel is being added at input port N (N>1),while input port 1 is used for the passthrough traffic from the lineinput port. These rules are based on a hierarchy of states as follows:

1) PT-N (Highest Priority)

2) BD

3) PTA-N

4) B (Lowest Priority)

State “PT-N” takes priority over all other states. For example, achannel that is set to the “BD” state that receives a “PT-N” requestwill switch to the “PT-N” state; whereas a channel that is set to the“PT-N” state that receives a “BD” request will remain in the “PT-N”state. This hierarchy gives priority to provisioned signals overprovisioned unused passthrough channels that are adjacent to signalchannels. Additionally, this hierarchy gives priority of provisionedunused passthrough channels that are adjacent to signal channels overunused channels without adjacent signals. More general rules can easilybe derived by those skilled in the art.

With these simplified rules, each WSS can set the state of each channelgiven the same provisioning request as a current WSS. Furthermore, thisnew provisioning method supports a self-provisioning WSS based onchannel powers as described in US Pat No 2010/0221004.

In one embodiment, for cases where a WSS is in state “PT-N” and a newprovisioning request occurs for “PT-M” where N is not equal to M, thenew provisioning request takes precedence. Similarly, where a conflictarises between “PTA-N” and “PTA-M” where N is not equal to M, the newprovisioning request takes precedence. In other embodiments, the priorstate is given precedence. Changing the configuration from “PT-N” to“PT-M” may affect traffic, so an optional warning to the networkoperator may be desired upon such a state change.

When a signal is removed from the network, the provisioning of the WSScan be adjusted using the state hierarchy described above. Where thestate was “PT-N” or “BD,” the state would revert to “PTA-N” where thereare adjacent provisioned channels, and the state would revert to “B”where there are no adjacent provisioned channels.

The full WSS state table, of which subsets are shown in FIGS. 4, 6, 9and 10, provides a single means of counting the number of bandwidthnarrowing events with respect to each provisioned signal. For eachchannel, software can work from the point where a signal is added (state“PT-2” for the case of a degree-2 WSS) to the state where a channel isdropped (state “BD”). For each WSS along that path, there is a bandwidthnarrowing event if the WSS of an adjacent channel is not in the samephysical state as the signal channel (despite a difference in thelogical state, such as a signal channel in state “PT-N” where adjacentchannels are in state “PTA-N”).

Therefore, the number of bandwidth narrowing events with respect to eachchannel (including both adjacent edges of each channel) can becalculated and reported, as well as used to minimize the number ofbandwidth narrowing events when determining the channel to which a newclient signal should be assigned. Furthermore, if there is a significantnumber of bandwidth narrowing events on one particular side of a signal,the bandwidth narrowing penalty of that signal can be reduced byslightly shifting the signal frequency to the other side of the centralfrequency.

In another embodiment, illustrated in FIG. 12, the WSS state table isused to select the channel (frequency/wavelength) to which a new clientsignal will be assigned for the purpose of minimizing bandwidthnarrowing. When the network management software is alerted to a newsignal request, the software can select the “ideal” channel inaccordance with the following method:

Each channel is examined in turn, beginning with step 1210. If anychannels remain (step 1215—YES), then the channel is examined in step1225 to determine whether it is available along the requested path. Ifthe channel at any node along the requested path is in a “PT-N” state(as distinguished from a “PTA-N” state), then the channel isunavailable, as it is already being used for a provisioned clientsignal. In that event (step 1225—NO), processing returns to step 1210 toexamine the next channel.

If the channel is available (step 1225—YES), then processing proceeds tostep 1230 to calculate the tentative new WSS state table for the newsignal path (e.g., as shown in FIG. 11, and in column 1030 of FIG. 10for Channel 193.9 Nodes 4, 5, 6, 7, 1 and 2). As shown in FIG. 11,adjacent channels must also be updated (as were columns 1040 and 1010 ofFIG. 10 for respective adjacent channels 193.8 and 194.0).

Processing then proceeds to step 1240 to calculate the total number ofbandwidth narrowing events for each affected channel in the tentativenew WSS state table. This includes not only the tentatively provisionedchannel, but also one or both of its adjacent channels if they are usedin the network to transmit signals. For the tentatively provisionedchannel (and its adjacent channels if they are used to transmitsignals), the physical state of the channel at the source node,intermediate nodes and destination node is compared to the updatedstates of each of its adjacent channels at those nodes. In thisembodiment, each difference in physical state (from each adjacent node)is considered a distinct bandwidth narrowing event.

The total number of bandwidth narrowing events for the tentativelyprovisioned channel is then compared to the total number of bandwidthnarrowing events for each adjacent channel used to transmit signals, andthe largest of these totals is saved. Note that we consider separately,on a per-channel basis, the total number of bandwidth narrowing eventsfor each of these channels, rather than adding these totals together,because the worst-case penalty occurs on the channel with the mostbandwidth narrowing events.

In other words, it is more important to reduce the maximum number ofbandwidth narrowing events on the worst-case channel than to reduce thetotal number of bandwidth narrowing events across multiple channels oreven the entire network. This is because bandwidth narrowing getsincreasingly worse on a given channel as the number of bandwidthnarrowing events increases (eventually leading to bit errors), butbandwidth narrowing on one channel does not affect bandwidth narrowingon other (particularly other non-adjacent) channels. For example, if biterrors began to appear after 7 bandwidth narrowing events occurred on agiven channel, then it would be preferable to have 100 channels eachwith 5 bandwidth narrowing events than to have 98 channels each with 2bandwidth narrowing events and 2 channels each with 10 bandwidthnarrowing events.

The number of generated “PTA-N” states is also calculated, on aper-channel basis—to “break a tie” in the event that more than onetentatively-provisioned channel generates the same lowest number ofbandwidth narrowing events. This information is also saved, andprocessing then returns to step 1210 to examine the next channel.

This process is repeated for each available channel, until no additionalchannels remain to be examined (step 1215—NO), at which point processingproceeds to step 1250 to determine which tentatively provisionedchannel(s) (after taking their adjacent channels into account, asdiscussed above) would yield the fewest number of bandwidth narrowingevents.

If more than one channel qualifies, then the channel that generates thefewest number of “PTA-N” states would be selected. If there stillremains more than one such channel, then additional “tie-breaking”factors could be considered, including the random selection of one ofthose channels. These tie-breaking factors are, in one embodiment,designed to favor the use of adjacent channels along the same path.Otherwise, signals could be provisioned where no adjacent channels arein use, resulting in a coarse distribution of provisioned channels thatcould unduly restrict channel availability at high channel counts.

The algorithm illustrated in FIG. 12 is designed not only to minimize(as well as significantly reduce) the bandwidth narrowing effects onadded channels, but to minimize the bandwidth narrowing effects of theadded channels on the other signal channels (e.g., adjacent channels).

While there can be some concern regarding crosstalk that arises fromusing adjacent channels for signal connections between the same nodes,those skilled in the art will recognize that current drop filters havesufficient isolation to render such crosstalk negligible.

The present invention has been described herein with reference tospecific embodiments as illustrated in the accompanying drawings.Although the WSS provisioning embodiments have been described for thecase of a unidirectional connection through degree-2 WSS devices in afiltered add configuration, the same concepts may also be applied tobidirectional connections, through higher degree WSS devices, and in WSSdevices in a filtered drop configuration or WSS configuration devicesused for both adds and drops. It should be understood that, in light ofthe present disclosure, additional embodiments of the concepts disclosedherein may be envisioned and implemented within the scope of the presentinvention by those skilled in the art.

1. A method for provisioning a channel on which a client signal will be transmitted along a path from a source node through at least one intermediate node before reaching a destination node of a WDM optical network, the method comprising the following steps: (a) selecting a signal channel and a path on which the client signal will be transmitted among a plurality of nodes of the network; (b) configuring an optical filter at each node along the path to reflect the state of the signal channel at that node; (c) determining, with respect to each node along the path, whether each of two channels adjacent to the signal channel is unused; and (d) configuring the optical filter at each node along the path by setting the state of each unused adjacent channel to the same physical state as that of the signal channel, (e) thereby effectively increasing the spectral passband of the client signal.
 2. The method of claim 1, wherein the physical state of the signal channel at the source node of the path is an “add” state, the physical state of the signal channel at each intermediate node of the path is a “pass through” state and the physical state of the signal channel at the destination node is a “blocking” state.
 3. The method of claim 2, wherein every channel is initially set to a default “blocking” state.
 4. The method of claim 1, wherein the state of each unused adjacent channel is set to a different logical state than that of the signal channel.
 5. The method of claim 1, wherein the level of attenuation of each unused adjacent channel is set to the same level of attenuation as that of the signal channel.
 6. The method of claim 1, wherein the optical filter at each node along the path is a wavelength selective switch.
 7. A method for minimizing bandwidth narrowing while provisioning one of a plurality of available channels on which a client signal will be transmitted along a path from a source node through at least one intermediate node before reaching a destination node of a WDM optical network, the method comprising the following steps: (a) for each available signal channel, (i) tentatively configuring an optical filter at each node along the path to reflect the state of the available signal channel; (ii) tentatively configuring the optical filter at each node along the path to set the state of each unused channel adjacent to the available signal channel to the same physical state as that of the available signal channel; (iii) determining the tentative number of bandwidth narrowing events from the perspective of the available signal channel, wherein a distinct bandwidth narrowing event would result for each channel which is adjacent to the available signal channel at a node along the path, and which has a physical state that differs from the physical state of the signal channel; (iv) repeating step (iii) from the perspective of each of the channels adjacent to the available signal channel that is being used to transmit signals; and (v) selecting the larger of the tentative numbers of bandwidth narrowing events determined in steps (iii) and (iv); (b) selecting the available signal channel that yielded the smallest number of bandwidth narrowing events selected in step (a) (v); and (c) provisioning the selected signal channel to transmit the client signal along the path.
 8. A method for minimizing bandwidth narrowing after provisioning one of a plurality of available channels on which a client signal will be transmitted along a path from a source node through at least one intermediate node before reaching a destination node of a WDM optical network, the method comprising the following steps: (a) calculating, for the provisioned signal channel, the number of existing bandwidth narrowing events with respect to each of the two channels adjacent to the provisioned signal channel, wherein a distinct bandwidth narrowing event occurs for each channel which is adjacent to the provisioned signal channel at a node along the path, and which has a physical state that differs from the physical state of the provisioned signal channel; and (b) shifting the frequency of the provisioned signal channel toward the frequency of the channel adjacent to the provisioned signal channel having the fewest number of bandwidth narrowing events, wherein the amount of the frequency shift is a function of the number of bandwidth narrowing events calculated with respect to each of the two channels adjacent to the provisioned signal channel. 